Presently, printing employing a CTP system has been conducted in printing industries, accompanied with the digitization of printing data. A printing plate material for CTP, which is inexpensive, can be easily handled, and has printability comparable with that of a PS plate, is required.
Particularly in recent years, a printing plate material has been sought which does not require any development employing a developer containing specific chemicals (such as alkalis, acids, and solvents), and can be applied to a conventional printing press. Known are a chemical-free type printing plate material such as a phase change type printing plate material requiring no development process, a printing plate material which can be processed with water or a neutral processing liquid comprised mainly of water, or a printing plate material capable of being developed on a printing press at initial printing stage and requiring no development process; and a printing plate material called a processless printing plate material.
In the CTP system, a process so-called “plate inspection” is required in present work flow as in a conventional PS plate. In the case where a printing plate material is developed and punched to form holes for mounting on a plate cylinder of a printing press, register marks are read through employing a dedicated device and their correct positions are determined. Therefore, it is necessary to have a reflection density difference between image portions and non-image portions of the developed printing plate material whereby the register marks can be read, and a printing plate material is required to provide a so-called development visualization property.
A printing plate material requiring no development process or a processless printing plate material to be developed on a plate cylinder of a printing press is required to provide an exposure visualization property, since it is punched after imagewise exposure to form holes for mounting on the plate cylinder.
In order to add this visualization property, there has been proposed a printing plate material possessing a hydrophilic overcoat layer, which is capable of being removed on a printing press, containing at least 20% by weight of a cyanine infrared absorbing dye whose optical density varies due to exposure (refer to Patent Document 1).
This printing plate material gives good exposure image visualization, but it is difficult to avoid color contamination caused due to development-on-press, since the high dye content of the overcoat layer to be removed on a printing press exhibits a high color density in either exposed potions or unexposed portions, whether a color density of the layer increases or decreases due to exposure.
An infrared absorbing dye contained in an image formation layer, which discolors on exposure, is commonly known, but in the printing plate material possessing an image formation layer containing such an infrared absorbing dye, increase of the difference in color density between exposed portions and unexposed portions is to increase a color density of the image formation layer at unexposed portions, resulting in color contamination due to the image formation layer at the unexposed portions during developing on a printing press (refer to Patent Document 2).
As a printing plate material developable on a printing press, there is known a printing plate material possessing an image formation layer containing materials colored via heating such as a leuco dye and a color developing agent, wherein only exposed portions or hydrophobic image portions color (refer to Patent Document 3).
This printing plate material, in which the image formation layer at non-image portions to be removed during printing, has a relatively low color density, and reduces degree of color contamination, compared with one employing a dye which discolors on exposure. However, there is problem in that image portions to have been colored partially have regions with a low water resistance, resulting in color contamination due to colored image portions.
On the other hand, a processless printing plate material is imagewise exposed employing an infrared laser with an emission wavelength of from near-infrared to infrared regions to form an image. The thermal processless printing plate material employing this method may be divided into two types; an ablation type printing plate material and a development-on-press type printing plate material with a heat melting image formation layer.
Examples of the ablation type printing plate material include those disclosed in Published Japanese Translation of PCT International Publication No. 8-507727, and Japanese Patent O.P.I. Publication Nos. 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773.
These references disclose a printing plate material possessing a support, and provided thereon, a hydrophilic layer and a lipophilic layer, either of which is an outer layer. When a printing plate material is imagewise exposed in which the hydrophilic layer is an outer layer, the hydrophilic layer is removed by ablation to reveal the lipophilic layer, whereby an image is formed.
However, this printing plate material produces a problem such that the exposure device is contaminated by the ablated material, resulting in low versatility to an exposure device, and specifically, the exposure device is required to be equipped with an additional suction device.
A printing plate material has been developed which is capable of forming an image without ablation, and does not require development treatment employing a special developer or wiping-off treatment. Known are, for example, a printing plate material disclosed in Japanese Patent Nos. 2938397 and 2938398 which comprises a hydrophilic layer or a grained aluminum plate and provided thereon, a photosensitive image formation layer containing thermoplastic particles, a water soluble binder and a light-heat conversion material, as the development-on-press type printing plate material; and a printing plate material which forms an image via heat generation of a light-heat conversion material by containing the light-heat conversion material in a hydrophilic layer (refer to Patent document 4).
Though such a printing plate material is capable of reproducing a comparatively high-resolution image, and is usable as a processless printing plate material, an insufficient exposure visualization property results. Even though a process of adding the above visualization property is also utilized, the visualization property is still insufficient, and as a result, it was difficult to improve a visualization property in the state of maintaining printing durability and on-press developability (particularly initial printability).
(Patent Document 1) Japanese Patent O.P.I. Publication No. 2002-205466
(Patent Document 2) Japanese Patent O.P.I. Publication No. 2000-225780
(Patent Document 3) Japanese Patent O.P.I. Publication No. 11-140270
(Patent Document 4) Japanese Patent O.P.I. Publication No. 2003-231374